Currently, it seems the following is not possible:
The reason is it produces
content: #3acfb6 instead of
content: '#3acfb6' — which makes sense.
However it seems there is no way to add quotes or otherwise convert
--color to string within
content: definition. Something similar is provided by
attr(), but not by
var(), which seems unfortunate.
I am not sure what would be the right solution here, but some way to do conversions would be very useful.